474. Ei'of. All man on the Hydroida; 



. While Haeckel has thus done good service to our knowledge 

 of the Hydroida in pointing out a genetic relation between the 

 JEginidcB and the Geryonida, his labours have been at least as 

 valuable in showing that the structure of the jEginidce is in all 

 essential points identical with that of the Geryonidce. He has 

 proved, for example, that the circular marginal canal, hitherto 

 denied to the ^ginida, is really present ; and there can accord- 

 ingly no longer be any difficulty in placing these IVIedusse in the 

 same group with the rest of the Hydroid or gymnophthalmic 

 forms*. 



It cannot, however, be overlooked that the position of the 

 ^ginidan buds is remarkable and anomalous ; for they are 

 borne by the solid tongue-like process which in Geryonia pro- 

 jects from the base of the manubrium into its cavity. In almost 

 eivery other known instance the somatic cavity of the Medusa-bud 

 is in communication with some part of the somatic cavity of the 

 Hydroid which produces it, while here such a communication is 

 impossible before the development of the mouth in the bud 

 shall enable the young ^Eginidan to receive nutriment from the 

 stomach-cavity of the Geryonia, Three cases, however, all 

 among the ^Eginidan Medussef, had been already described, 

 in which the young Medusse are formed as buds from the in- 

 ternal surface of the stomach-walls, and therefore, just as in 

 the present instance, these young buds could not have had their 

 somatic cavities in communication with that of the animal which 

 carries them. The buds must accordingly have been formed in 

 a very different way from that which takes place in the ordinary 

 cases of budding Medusse, — so different, indeed, that, were it 

 not for the competency of the observers who have described 

 them as cases of true budding, we should be disposed to regard 

 them as suggesting pai'asitism rather than gemmation. 



* The names of Crypto^arpa, Eschsch., Gymnophthalmata, Forbes, and 

 Craspedota, Gegenb., are each, as is- now known, inapphcable to certain 

 members of the group of organisms which they were originally intended 

 to distinguish. The inconvenience, however, arising from this fact may be 

 avoided by the use of the designation Hydroid Medusa, which would in- 

 clude under it not onl}^ those gymnophthalmic forms which are known to 

 proceed from polypoid trophosomes, but also such as have not been as yet 

 so traced. " Medusa," however, must be understood as a term rather than 

 as a systematic name. 



f Gegenbaur, in ' Generationswechsel,' p. 56, Cunina proUfera; Kefer- 

 stein and Ehlers, in ' Zoologische Beitrage,' 1861, jEgineta yemmifera; 

 and Fritz Miiller, in * Wiegm. Arch.' 1861, Cunina Kollikeri. 



